Method and apparatus for the inspection of paint films



April 11945. F. DIEFFENBACH 2,373,104

METHOD AND APPARATUS FOR THE INSPECTION OF PAINT FILMS Filed July 13, 1942 Fig-7.

D D/ FFENBACH ATTORNEYS 5 +50 mun Patented Apr. 10, 1945 UNITED STATES PATENT OFFICE 2,373,104 v METHOD AND APPARATUS ron r m msrao- TION or 1mm FILMS Ferd Dieifenbach, Greentree, Pa. Application July 13, 1942, Serial No. 450,785

3 Claims. (or. 83-14) (Granted under the act of'March 3. 1883, as

amended April 30,1928; 370 O. G. 757) The invention described herein may be menufactured and used by or for the Government for governmental purposes, without the payment 'to me of. any royalty thereon.

This invention relates generally to inspection apparatus but more particularly to a method and apparatus for facilitating the inspection of paint films.

observation through a microscope; said means and method being particularly applicable to transparent, translucent, and low pigment'vehicles.

Another object of the invention is to provide a method and means for tentatively'identifying unknown vehicles and forestimating within ten days or less the performance of said vehicles.

Another object of the invention is to provide a a method and means by which the treatment of a film may be studied, such as the structural change in heat treatment, addition of driers, change in oil lengths, types of resins, etc. Most paint vehicles have been found to be very heterogeneous when studied by this method, with cellular structures and cell boundaries, which cell boundaries attribute very definite properties to the paint vehicle, which in turn permits logical explanations for many types of paint performances.

Another object of this invention is that it provides a method and means of analysis which permits the scientific selection of pigments for a specific vehicle, the study of transfer. of liquids through a paint film, the adhesion of varioustypes of paints, the eil'ects on change of structure of paintas it ages, and the physical-chemical theory of vehicles.

Another object of the invention is to provide a method and meanswhich permits the study of blistering, the orientation of paint particles, and of the wetting and selective absorption of pigments in a vehicle, and also floating of pigment.

By the method and means disclosed herein, it is proved that many types of blistering are caused elements in the apparatus used whereby various desirable effects will be obtained, all as will be more fully described in the following specification, and shown in the drawing, and will be further pointed out in the appended claims.

Referring to the drawing in which like parts I are indicated by similar reference characters:

Fig. 1 is a front elevation of a microscope showing the arrangement of light sources for the illumination of paint film;

Fig. 2 is a microscopic enlargement showing the film structure of a linseed oil-China-wood oilphenolic resin varnish, illustrating the hexagonal cellular structure of the film;

Fig. 3 is a microscopic enlargement showing the film structure of China-wood oil with elongated cellular structures of a wrinkled nature Fig. 4 is a microscopic enlargement showing the 20 film structure of a glycerolphthalate varnish;

Fig. 5 is a microscopic enlargement showing the film structure of linseed oil;

Fig. 6' is a microscopic enlargement showing the film structure of a phenolic resin, China-wood oil-linseed oil .varnish identical to the one shown in Fig. 2, with the exception that a small amount of zinc yellow has been added to the varnish;

. erably mounted within small cylindrical housings Fig. 7 is a microscopic enlargement showing a steel test panel which was painted with phenolic paint.

In order to describe the device in detail, reference will be had to Fig. 1, in which the reference numeral Ill indicates an assembled microscope,..ll the base, l2 the stand or supporting column, l3 and I3 the telescopic cylinders which retain respectively the eye-piece l4 and the ob- .lective lens l5, IS the focusing device for said cylinders, I! the stage upon which the object under examination is mounted, and I8 the adjustable reflecting mirror which directs a light beam through an opening I! in the center of the stage. In conjunction with the microscope, two light sources are utilized to properly illuminate a paint coated glass plate or microscope slide 20 which is placed on the stage.

The light sources or lamps 2| and 2| are provided with reflectors 22 and 22', and are pref- 23 and 23' forming spot lamps 24 and 24' which are rovided with adjustable supports 25 and 25' i and lenses 24 and 26' suitable to concentrate the by transmission of a liquid through the cell body of vehicles, which transmission may be reduced or stopped by proper pigmentation.

Another object of the invention is the provislon of a comparatively simple and inexpensive LWQBYIGQQLQIQ character referred to. which will be dependable 'andextremely effective in operation. Other objects of the invention consist in the provision for' the arrangement and combination of .rays in the direction desired. These lamps 24 and. 24' are placed so that the rays emanating from lamp 24 may be directed-on the mirror l8 and from the mirror reflected through the opening IS in the center of the stage I! and through paint. coated transparent plate or slide 20 which is placed on the stage. The other lamp 24' is so placed that the rays emanating therefrom will e directed on one edge of the paint coated plate and will also pass across the upper surface thereof.

With the lamps placed in the positions asshown in Fig. l, the film of paint on the glass plate or slide 20 will be illuminated so that the physical structure of the film may be observed through the microscope.

The method of inspecting and analyzing paints with the apparatus described consists first in preparing slides or plates 20 which are subsequently used for the study of paint film structures. .The film to be studied is coated on the microscope slide 20 preferably by dipping, after which the film is allowed to dry. When the film has dried, the slide or plate is immersed in a liquid similar to the material in which the paint will be used such as water for atmospheric exposures, salt water for sea exposures, etc. As the result of this immersion, the structure of the paint film will become defined by the selective absorption of the immersion liquid into and through the paint film, The slides are examined daily under the microscope at about 100 diameters, during the immersion period, and the structure of the paint film is usually revealed within about three to five days. The structure is visible and outlined by virtue of refraction differences in the paint film due to the diiierential light absorption and transmission through the paint film resulting in irregular trapped liquid in the interface between the paint film and the glass slide. Dyes may also be included to help define the structure.

It will be noted that most films have a cellular structure in which the cell proper has relatively large water transmission properties, while the cell boundary is relatively impervious to water.- Fig. 2 shows a film structure of a linseed oil-Chinawood oil-phenolic resin which paradoxically has a hexagonal cellular structure which is probably influenced by the hexagonal phenol ring in the resin. It is believed that in this case the resin is in solution with the China-wood and linseed oils with the oils forming the cells proper and the resin forming the cell boundaries. The same structure influences paint films and their adhesion which may be noticed in Fig. '7, which shows this hexagonal structure in a paint failure.

The film structure of the China-wood oil with elongated cellular structures of a wrinkled nature. as shown in Fig. 3, demonstrates the open chain structure of China-wood oil. The structure of China-wood oil indicates the reason for the wrinkling of China-Wood oil bearing varnishes.

In the schematic sketch showing the film structure of a glycerol pthalate varnish, Fig. 4, the relatively small size of the cells compared with the phenolic varnish explains the reason for the chosen which will make these cells less permeable to the transmission of liquid which leads to an better adhesion of the glycerol phthalate varnish Thus, in accordance with the description given above, the. means and method, which form the subject matter of this invention, provide a rapid process for the development, identification, and study of translucent or transparent paint films.

The schematic sketch (Fig. 6) shows the film structure of a phenolic resin-China-wood oil-linseed oil varnish identical to the one shown in Fig. 2, with the exception that a small amount of zinc yellow is absorbed primarily by the cellular areas as shown, which indicates that the cellular sections of this type of vehicle have a very much higher pigment absorption than the cell boundaries. By virtue of this test, pigments can be early failure of the paint.

The method and apparatus described herein provide a technique for the study of paint films and are also important in the study of paint adhesion in that the adhesion is destroyed when liquid passes through the cells in the paint film.

The method and apparatus also permit the study of the physiochemical theory of vehicles as illustrated in Fig. 7 which shows a steel test panel which was painted with phenoline paint, the first coat being an iron oxide primer and the second coat being aluminum paint. The aluminum paint failed by peeling on all surfaces except the clearly defined hexagom which represent the cell boundaries of the phenolic vehicle. This same structure can be readily observed with the means and method described herein. From these results it is believed that phenolic varnishes are slutions of the phenolic resins in the drying oils with the resin forming cell boundaries in hexagonal form as influenced by the benzine ring structure with the oil as the cell proper.

Having described my invention, what I claim as new and wish to secure byLetters Patent is:

l. A method for microscopically examining paint films consisting in forming a film of dry paint on a microscopic slide, developing the cellular structure of the paint by immersing the slide with its film in a fluid similar to that in which the paint is to be used, and retaining it in the immersion fluid for a period of '72 to hours whereby the difference in the fluid absorption and transmission properties of the paint cells and their boundaries effect a delineation of the cellular structure of the paint film, and then visualizing the delineated cellular structure through a'microscope by differential transmission and reflection of light rays directed angularly through, and longitudinally through and above the paint film.

2. A method for examining the structure of a paint coated surface microscopically comprising, successively applying and drying coats of paint to a miscroscopic slide, immersing the paint coated slide in a fluid similar to the medium in which the paint is to be used and retaining it in the immersion fluid for a period of 72 ,to 120 hours, whereby the difference in fluid absorption and transmission properties of the paint cells and their boundaries effect a delineation of the cellu- I lar structure of the paint films and then yisualizing the delineated cellular structurethrough a miscroscope by differential transmission and refiection of light rays directed angularly through and parallel to the surface of said slide.

3. A method for microscopically determining the physical structure of paint comprising the following steps, coating a surface of a microscopic slide with hardened superimposed layers of the paint to be examined, submerging the coated slide in a fluid similar'to that in which the paint is to be exposed and retaining it in the immersion fiuid for a period of '72 to 120 hours, to eflect the absorption of the fluid within the cells and cell boundaries of said layers, whereby the fluid absorption and transmission properties of the cells and their boundaries effect a delineation of the cellular structure of the paint films, and then rendering the cellular structure visible by differential transmission and reflection of light rays directed angularly and longitudinally through said slide for microscopic examination.

FERD DIEFFENBACH. 

